Preoperative Precautions
Preoperative calculations of the IOL require painstaking
care, and if not performed correctly can result in
postoperative refractive error disasters. The range
ofpostoperative refractive error that patients will
be happy with today is narrow. They expect to see
well uncorrected immediately after cataract surgery;
to meet this expectation, surgeons must determine
the correct IOL power measurement.
To achieve that accuracy,
you must avoid several potential pitfalls in the preoperative
workup: Perhaps most vulnerable to surgeon error are
axial length measurement and keratometry, essential
factors in IOL power calculations. (For more information,
see "Nine Ways to Improve Your IOL Calculations," April
1999.)
Complicating the issue
even further is that our cataract patient population
now includes individuals who have undergone refractive
surgery. A significant, and rapidly growing, number
of patients seen every day in U.S. cataract practices
have had radial keratotomy, astigmatic keratotomy or
a laser refractive procedure. The resultant comeal
changes can be challenging when determining IOL power.
In general, the most
efficient way to accurately calculate the intraocular
lens power in these eyes is to identify the keratometry
prior to the refractive procedure, as well as the amount
of refractive error corrected at that time. As a general
principle, the current axial length measured is included
in the surgeon's favorite IOL calculation formula.
The original average keratometry is used after the
amount of refractive error previously corrected is
subtracted. For example, if the patient's original
keratometry reading before the refractive procedure
was 45 D, and that patient had a PRK to correct -6
D, the effective keratometry reading for the IOL calculation
formula would be 45 - 6, or SOD (average keratometry).
Accurate A-scans are
also critical, and achieving the correct measurement
can be challenging, especially for newly trained technicians,
in highly myopic patients who may have staphylomas
that can cause variable measurements. Very high myopic
or hyperopic axial lengths challenge even the most
sophisticated IOL calculation formulas. Most of these
measurement problems can be avoided by using the surgeon's
favorite algorithm routinely; experience with one formula
can maximize the accuracy of the results.
Intraoperative
Surprises
A variety of intraoperative problems can challenge
cataract surgery. Though most are not serious, their
management does significantly affect the outcome of
the procedure. Important factors that need to be assessed
at the end of cataract extraction and before IOL implantation
are:
- Stability and structure of the cataract wound.
- The condition of the anterior capsulotomy.
- The amount of the zonular support.
- Posterior capsular integrity.
If there is no problem
with the wound, the anterior capsule has no tears,
there is good zonular support and the posterior capsule
is intact, then a foldable intraocular lens can be
implanted and unfolded within the capsular bag. If
a problem exists in one of these areas, you must reconsider
the decision to implant an IOL in the bag.
One rare but significant
problem is an artifact, flaw or damage in the intraocular
lens. Every surgeon should carefully evaluate the structure
and integrity of the intraocular lens prior to its
implantation, even in the "20th case of the day."
Take foldable lOLs,
for example. Most of these lenses are folded in the
operating room before implantation. The folding process
itself can bend or break the haptics and scratch, break
or cause permanent striae in the optic. Noting damage
before beginning to implant the lens or before completing
implantation in the capsular bag allows the surgeon
to replace the damaged IOL.
If the damage is seen
after the lens is implanted, then the surgeon faces
explanting the now unfolded intraocular lens from the
anterior chamber or the capsular bag and replacing
it with another lens. If you encounter this scenario,
consider these options: Either enlarge the incision
(if it is scleral), or work through a second scleral
incision, or bisect the lens into two pieces for easier
explantation.
An abrupt release
of the haptic and/or optic from the IOL injector or
other folding instrument is another possible problem
with foldable lOLs. This uncontrolled release can cause
such severe trauma to the posterior capsule that IOL
implantation is compromised. In these cases, it is
prudent to avoid vitreous prolapse by limiting intraocular
manipulations until you have devised an appropriate
surgical plan. If vitreous prolapse is present, I suggest
completing a thorough anterior vitrectomy, and then
deciding whether the unfolded IOL can be placed in
the capsular bag or if it can be stabilized in the
sulcus. If you decide that the foldable IOL, now unfolded
in the eye, will not be stable within the residual
capsular bag or the sulcus (usually the case with lenses
of smaller total diameter), remove the IOL and replace
it with a larger total diameter lens that can be sulcus-fixated.
If there is not sufficient capsular support, an anterior
chamber intraocular lens may be inserted. Also consider
a posterior chamber, scleral-fixated intraocular lens.
If zonular dehiscence
occurs, IOL implantation can be quite challenging.
One option is to attempt sulcus fixation. Another is
the use of a capsular expansion ring. Several companies
market these rings, which usually are made of PMMA
and are very easy to implant within the capsular bag.
Their total diameter varies between 11 and 13 mm. They
expand once placed within the capsule and provide better
bag stability for posterior chamber intraocular lenses,
especially in cases with poor zonular support or compromised
capsular bags. I have successfully used these rings
several weeks postoperatively to help correct IOL subluxation
due to partial zonular dehiscence. |
